1. Field of the Invention
This invention relates to an improved high-temperature electrically regenerable electrochemical system. It more particularly relates to an alkali metal-transition metal chalcogenide secondary cell or battery providing long cycle-life at high energy densities and having a high coulombic efficiency under conditions of repeated cycling.
2. Prior Art
High energy density batteries are of particular interest for application as a source of power for an electric vehicle and for load leveling in the electric utility industry. Initially, the interest was directed toward the lithium-sulfur cell using a molten halide; see M. L. Kyle et al., "Lithium/Sulfur Batteries for Electric Vehicle Propulsion", 1971 Sixth Intersociety Energy Conversion Engineering Conference Proceedings, p. 38; L. A. Heredy et al., Proc. Intern. Electric Vehicle Council 1, 375 (1969). Such lithium-molten salt batteries using sulfur positive electrodes when fully developed could provide an energy density of greater than 100 watt-hr/lb. Were a cycle life of 2500 cycles and an operating life of 10 years attainable with these batteries, they could satisfy all the requirements of electric power peaking, which is of great interest to the electric utility industry for providing off-peak energy storage and load leveling.
It has been found, however, that long cycle life is difficult to attain with such high-temperature molten salt batteries containing a sulfur electrode because of the gradual loss of the active sulfur material from the positive electrode compartment at these elevated temperatures. Sulfur loss generally occurs by vaporization of the sulfur or by dissolution of intermediate discharge products (polysulfide ions) in the molten salt electrolyte followed by diffusion from the positive electrode compartment through the bulk of the electrolyte to the negative lithium electrode.
To eliminate some of these problems, it has been proposed (U.S. Pat. No. 3,898,096 issued Aug. 5, 1975) to use certain selected transition metal chalcogenides as the positive electrode material in lieu of elemental sulfur. The preferred positive electrode materials are copper sulfide, iron sulfide, nickel sulfide, and nickel oxide. The patent teaches that the positive electrode materials, which are in solid form at the operating temperature of the molten salt electrolyte battery, must be made readily available in a finely divided form presenting a high specific surface.
Several methods are suggested for presenting such a high specific surface of the positive electrode material. In accordance with one suggested method, a lattice of porous graphite is used, and the lattice is impregnated with the positive electrode material using a slurry of such material in a volatile liquid. The porous graphite lattice then is baked to evaporate the volatile liquid, leaving the positive electrode material in the form of fine particles distributed throughout the interstices of the porous graphite lattice. The other suggested methods are substantially the same as those utilized in the prior art for cathodes which employed elemental sulfur as the positive electrode material.
It has been discovered, however, that certain problems are encountered when a transition metal chalcogenide is used as the positive electrode material, which problems are not present when such material is elemental sulfur. More particularly, during discharge of a battery which utilizes iron sulfide as the active cathode material, the iron sulfide reacts with lithium to form elemental iron and lithium sulfide. The iron and lithium sulfide so formed occupy a volume approximately twice that of the original iron sulfide. Thus, sufficient void space must be left in the matrix to allow for such expansion in volume. The iron sulfide, iron, and lithium sulfide are solid at the operating temperatures of the battery. Therefore, unlike sulfur, which is liquid at the operating temperature of the battery and can move throughout the substrate to evenly distribute the loading, the use of a metal sulfide can result in high localized loading of the substrate. Such high localized loading can result in a physical breakdown of the substrate structure.
More recently, it has been suggested in U.S. Pat. No. 3,925,098 that the transition metal chalcogenide be loaded into a porous pliable felt matrix formed from resilient carbon fibers or filaments retained in a rigid structure. Such method overcomes the prior art problems of high localized stresses and the prior art rigid substrates. However, the utilizable ampere-hour capacity per cubic centimeter of electrode prepared in such a manner is still low, generally in the order of about 0.6 ah/cc using FeS.sub.1.5 (an equimolar mixture of FeS and FeS.sub.2), for example. Obviously, there is still a need for an improved positive electrode utilizing such transition metal chalcogenides as the active material.
Two approaches generally have been followed in the construction of a negative lithium electrode for use in an electrical energy storage device, such as a rechargeable battery, particularly one employing a molten salt electrolyte. In one approach, the lithium is alloyed with another metal such as, for example, aluminum to form a solid electrode at the operating temperature of the cell. In the other approach, liquid lithium is retained in a foraminous metal substrate by capillary action. Heretofore, the latter approach has been preferred because it offers higher operating cell voltages and therefore potentially higher battery energy densities. Certain problems are encountered, however, when it is attempted to retain molten lithium in a foraminous metal substrate. More particularly, most metals which are readily wetted by lithium are too soluble in the lithium to permit their use as the metal substrate, whereas most metals structurally resistant to attack by molten lithium are poorly wetted by the lithium when placed in a molten salt electrolyte.
It has been suggested that metals structurally resistant to attack by molten lithium may be wetted by immersion in molten lithium maintained at a high temperature. However, the structure so wetted by lithium at these higher temperatures usually undergoes progressive de-wetting when used as the negative electrode in a secondary battery containing a molten salt electrolyte maintained at the substantially lower temperatures at which such a battery operates. Thus, after operation of the battery for a number of cycles, it has been found that lithium no longer preferentially wets the substrate, the electrode progressively losing capacity. Various methods have been proposed in an attempt to overcome this problem. See, for example, U.S. Pat. Nos. 3,409,465 and 3,634,144. None of the proposed methods have proven entirely satisfactory.
The use of a solid lithium alloy as taught by the prior art also is not without problems. More particularly, lithium-aluminum alloy, for example, is approximately 300 millivolts more positive than liquid lithium. Thus, electrochemical cells utilizing lithium-aluminum alloys as electrodes are not able to achieve the same potentials as those utilizing liquid lithium electrodes. Further, in a molten salt electrolyte, the lithium-aluminum alloy electrode expands and contracts greatly during charging and discharging of the electrochemical cell. Still further, lithium-aluminum alloys generally are limited to a lithium content of less than about 30 wt.%.
Various other materials have been suggested for use as an alloy with lithium to form a solid electrode. In U.S. Pat. 3,506,490, for example, it is suggested that the lithium be alloyed with either aluminum, indium, tin, lead, silver or copper. However, none of these materials have been proven to be completely satisfactory. More particularly, these other suggested materials, such as tin and lead for example, form alloys containing lesser amounts of lithium than does aluminum, and thus have a still lower capacity (ampere-hours) per unit weight of alloy. Further, the potential of these other alloys compared with liquid lithium is more positive than that of a lithium-aluminium alloy; thus, alloys of such other materials are less desirable. Other patents relating to solid lithium anodes include U.S. Pat. Nos. 3,506,492 and 3,508,967.
More recently, in U.S. Ser. Number 605,691 filed Aug. 18, 1975, now U.S. Pat. No. 3,969,139, and assigned to the Assignee of the present invention, there has been suggested an improved lithium electrode and an electrical energy storage device such as a secondary battery or rechargeable electrochemical cell utilizing such electrode. The improved electrode comprises an alloy of lithium and silicon in intimate contact with a supporting current-collecting matrix. The lithium is present in the alloy in an amount from about 28 to 80 wt.%.
Various supporting current-collecting matrices are suggested. It is taught that the support and current-collecting capability may be provided by a single structure, or the support provided by one structure and current-collecting capability by another structure. The matrix for impregnation with lithium-silicon alloy is disclosed as a porous substrate having an apparent density from about 10 to 30% of that of the base material and a median pore size within the range of from about 25 to 100 microns. A particularly preferred form of such a substrate is formed from woven or non-woven wires pressed together to a desired apparent density and then sintered. It was noted by Patentee, that during some of the early tests, the supporting current-collecting matrix underwent a physical disintegration. At that time it was not known whether such deterioration was the result of a chemical reaction, or if it resulted from a physical expansion of the alloy. More recently, it has been determined that the alloy expands and contracts upon charge and discharge cycling. As in the case of the positive electrode, when the matrix is formed from a material having a sufficient size and strength to withstand the expansion forces of the alloy, the ampere-hour capacity per cubic centimeter is below that desired.
Thus, a need still exists for a lithium electrode structure which would retain its capacity upon continued cycling when used as a negative electrode in an electrochemical cell, which preferably would have a potential as close to liquid lithium as possible, and which would maintain its dimensional stability during charging and discharging of the cell.